Zinc metallurgy



A. y L. `J. QUENEAU ZINC METALLURG'; Filed uay 17, 1941 Feb. 22, 1944.

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` WNS gl MMHWJML. A :Ally www@ Pnented Feb. 2z, 1944 UNirED s'rirrrasgPATENT 'ori-'ica 2,342,368 l l I znw METALLURGY Augustin Leon JeanQueneau, Peapack, N. J.

Application May 1v,-1941,'seriai No. 394,009 f 4 claims. (cl. 'z5-2e)Inainc metallurgy, it is not an entirely new conception to proceed bywhat may be termed a "fiashyreduction operation for winning zinc fromzinciferous metallurgical materials.

In accordance with such flash reduction" procedure, oxidic zincmaterial,I together with carboniferous reducing agent, is showeredthrough an environment which `is heated to temperatures above thereduction and volatilization temperatures of zinc, the reduction takingplace in the freely falling charge while the particles are in suspensionin the environment. The carbon must be present in suillcient excess tocontact efficiently with the zinc oxide particles during their fall, sothat reduction and volatilization of the zinc will proceed before thecharge reaches the bottom of the reactant environment.

In carrying out the flash reduction process, it

has been the practice to shower the zinc oxide-- carbon mixture througha vertical columnar retort composed of a monolithic structure ofessentially silicon carbide. The retort is fired externally, andpreferably a reducing environmentis maintained in the retort in order tofacilitate the reduction of the zinc oxide while it is freely falling.Propane gas has been employed for the maintenance of this reducingatmosphere, a propane atmosphere apparently tending to reduce the amountof blue powder produced during the operation. The formation of this bluepowder has been one detriment to the flash reduction operation as hasbeen practiced heretofore. This has led to various proposalsx forconditioning and scrubbingthe emergent zinc stream .as it passes l tothe condenser to remove various vsuspensoids in the gas stream whichappear to act as nucleiA for blue powder production.

Additionally, it has been proposed to increase the reduction rate byusing activated, or alpha, carbon as the reducing agent.' In Vpriorprocedure, the elciency of the process is found to be increased thereby.l

These specialized provisions of the'prior practice are required becauseof certain existent factors:

l. The presence of suspensoids in the exit gases is due to entrainedsoiled particles from the of carbon arises because of imperfect physicalcontactbetween the particles oiA zinc oxide and the particles of carbon.

`3. This imperfect physical contact requires forefilciency a speeding upof the reduction speed,-

which is attained by the use of highly reactive (activated) carbon,materially increasing the cost of the procedure.

4. The life of the silicon carbide "ash" column is not sufficientlysatisfactory to make a flash reduction procedure competitive withrespect to costs,. with standard vertical retort operations.

In accordance with the present invention, the cause of theunsatisfactorily shortened life of the 'silicon carbide "flash columnsemployed in flash reducing operations as practiced prior to the presentinvention, now is found to be due to the presence of materialsassociated with zinc oxide, which even though present in sulcicntlysmall amounts so as to enable the zinc oxide .being employed to beclassed as substantially pure zinc oxide, nevertheless, under theconditions existing in the ash column where the entire charge is beingmade up of continuously y swirling particles being continuouslyintroduced j into the flash column, which particles` intimately engagethe hot silicon carbide surfaces of the column and come into wipingcontact therej with, such materials under these ideal conditions forreaction attack both the silicon carbide of the column and the binderemployed to bond the silicon carbide during the manufacture of thecolumn, lthe-resulting reactions causing a markedly quick disintegrationand deterioration of the column. Such materials are found to be.

importantly, certain slag-forming materials normally present with zincoxide produced from zinc ores or concentrates, among which materials maybe mentioned, importantly. lime, magnesia, iron bide bricks whichoxides, lead, cadmium, silver and alkalies, which under the conditions..maintained in the retort,-

react with the silicon. carbide to produce silicates' of the respectivemetals.

While the silicon carbide shells which form the fiash"column in theusual type thereof are found to be destroyed with marked rapidity by thepresence of even small amounts of such materials A in the zinc oxidebeing reduced, the same type of attack is observed inthe case of thesilicon carve been employed in the construction of vertical retortsusing briquetted charges of zinc oxide 'and carbon, although in suchinstallations, the attack is much slower, as

the conditions for reaction with silicon carbide are much less favorablethan are the conditions ex,-

istent in a flash reduction column. However, even in the carborundumbrickwork of vertical zinc retorts, in time there becomes a veryappreciable attack on the surfaces of the brick that are exposed to thebriquetted charges being processed.

The procedure of the present improved process is found to obviate these,and other, difficulties which have made impracticable a process whichshould be an attractive procedure from the standpoint of costs, as wellas the yield and purity of the condensed metal. l

Certain features distinguish the present invention:

A. A careful purifilation of the zinc-bearing materials to eliminateassociated impurities, thereby yobviating the necessity for specialconditioning or scrubbing ofthe zinc-bearing gases passing from theflash retort to the condenser.

B. The production and maintenance of a homogeneous and intimate contactof the reactive carbon with the oxidic component of the charge,

to eliminate the necessity of utilizing specially prepared activated oralpha carbon.

-tage in the operation of other zinc metallurgical processes, includingthe usual type of vertical retort practice. v

The invention will be understood more readily by reference totheaccompanying drawing, which illustrates a diagrammatic flow-sheet of theprocess.

'I'he present process may be divided into several cooperating parts, theinterrelationship of which will become apparent from a consideration ofthe accompanying drawing which illustrates a` diagrammatic flow sheet ofthe entire process. These cooperating parts are indicated in thediil'erent stages set forth in the description which followshereinafter.

Preparation of the charge One of the important features of the presentinvention is the preparation of the charge for the reduction.

.Zinc ores occur in nature mixed with gangue materials and usually carrytoo low a percentage ,ofzinc to allow for economicaltreatment forreduction to metal or chemical compounds. The crude ores, therefore, areprocessed so as to; yield high grade concentrates; in the case of.sulphide ore, of 60 per cent zinc or better. ,'Even these high gradezinc concentrates contain substantial amounts of contaminating chemicalelements or compounds, such as sulphur, iron, silica, lime,

cadmium, lead, etc.

In many instances, it is of economic interest to increase further thezinc contents ofl the mill concentrates mentioned above. Thus, thesulphur may be eliminated by suitable oxidizing treatment, such asroasting; In accordance with the present process, there are concentratedfurther the zinc contents of roasted zinc concentrates by the furtherelimination ofthe remaining vimpurities by a suitable pyro-metallurgicaltreatment described herein'.

2 as-iases The roasted zinc concentrates containing as main impuritiesiron and/or manganese oxides, lead sulphate, and small amounts ofcadmium and precious metals, in accordance with the present inventionare mixed with a suitable proportion of carbonaceous material, such ascoke 6r anthracite coal, or other carbonaceous material high in fixedcarbon, and the resulting mixture is fed into an inclined rotary kiln ofthe general type used inthe well-known Waelz process for producing zincoxide. The heat gradient required for .the operation is maintainedprincipally by the combination of the fuel addition to the zinc orecharge and, in addition, if required, by a burner supplying additionalheat through the burning of powdered coal, fuel oil, or other suitablefuel, the selection of the fuel supply for the burner being largely aquestion of convenience and costs.

In its passage through the rotary kiln, the zinc oxide is reduced withinthe body of the charge, tumbling and rolling in contact with the lowerand rising part of the kiln lining, principally in accordance with thereaction:

As there is of lnecessity an excess of carbon present in the kilncharge, a.' secondary reaction also takes place:

COa-I-C-)ZCO 'I'he metallic zinc vapors, together with the carbonmonoxide issuing from the highly heated charge, burn freely in contactwith the current of highly heated air admitted 'at the discharge end ofthe kiln. It may be noted in passing that that heat liberated by thecombustion of zinc vapor to zinc oxide and of the carbon monoxide tocarbon dioxide is substantially equivalent to the amount of heatrequired for the reduction of' zinc oxide in the roasted ore, withallowance for the heat which is carried away from the kiln by the gasesof combustion, and also for the heat that must be supplied to theradiating outer surface of the kiln.

Now, as the vapors of the reduced zinc are burned, zinc oxide isre-formed. But now this zinc oxide is liberated almost completely freedfrom gangue impurities, except a small amount thereof that is carried asne dust by the gas stream sweeping through the kiln to its exit.However, the resulting zinc oxide usually contains some lead, cadmium,and silver, since lead is reduced with the zinc and volatilizes as leadAretaining the precious metals in a complex ironcopper matte which is anactive absorber of gold and silver. If copper is absent in the zinccalcines, an addition of copper-bearing material to the charge may befound to be advantageous.

The initially puried zinc oxide is collected in any suitable way, as intextile bags, or in -a Cottrell precipitator. This zinc oxide containssubstantially all of the zinc content of the roasted ore charge with itslead and cadmium contents. It also includes a small amount of fineparticles of gangue, of carbon, and also a comparatively large volume ofsulphur dioxide and other gases s occluded and adsorbed on the zincoxide particles, so that further treatment of the zinc oxide is requiredfor the elimination of the lead. cadmium, silver, and the occludedgases.

For this purpose, the crude zinc oxide is mixed thoroughly with a halidecompound, preferably sodium chloride. The resulting mixture is fed ontothe bed of a blast sintering machine, such as a Dwight-Lloyd machine,with the salt presentV in the charge in slight excess of thestoichiometric requirements of the lead, cadmium, and silver. The chargeis sintered,

This sintering removes al1 of the contaminating metals, lead, cadmium,and silver, as chlorides and/or sulphates, together with the Whole ofany sulphur present. These contaminants are carried /away from thesinter bed by the gases sweeping at high velocity through the bed. Theresulting gases pass through a suitable Cottrell precipitator in whichthe metal compounds are collected. i

The sintering operation is carried on in such a way as to produce alight porous sintered product, this being obtained by adding to thecharge of the sintering machine a strict minimum amount of xed carbon,such as coke or anthracite coal, for minimizingI the maximum temperaturereached in the bed of the sinteringmachine.

The resulting sintered product consists of substantially pure zinc oxidewhich is highly suitable for further metallurgical treatment for theproduction of high grade zinc metal by flash or suspensionv reduction orin a vertical continuous retort, or other procedure.

The residual gangue matter in the rotary kiln charge is discharged atits lower end, together -with any excess of carbon that may be present.

If the kiln charge contained suilicient values of precious metals,together with an adequate amount of copper, the resulting complexironcopper matte carrying the precious metals may be separated readilyfrom the otherwise valueless slag by crushing and treatment by the usualoredressing methods,'such as tabling, but preferably by flotation.

It will be seen, therefore, that from a complex zinc ore carrying ganguematerials, such as iron, manganese, silica, lime, etc., togetherwithadditional metal values, such as lead, cadmium, and

tions amounting to from about 15 parts by weight to about 20 parts byweight.

The mixture then is passed through a steam jacketed pug mill from whichthe thoroughly lneorporated mixture is extruded in the form of a hollowcylindersix inches in diameter with one and one-half inch centralperforation, and cut into twelve-inch lengths orbriquettes. 'I'hesebriquettes are piled on a platform car and run into a tunnel furnace,heated indirectly, in which there is maintained a maximum temperature offrom about 600 C. to 800 C., the briquettes being brought slowly to fulltemperature until al1 volatile ymatter has been expelled and thebriquettes are coked.

` The coked briquettes are crushed all through 4a one-inch screen, thecrushed product then being fed to a rodmill operating in a closedcircuit with an air classifier to give a iinal product,- about 85 percent of which passes through a 20D-mesh screen. I

The particles of this product consist principally of puritled zinc oxidecompletely permeated with xed carbon deposited during the distillationand coking of the briquettes. There is provided thereby completelyintimate contact between the reducing material and the zinc oxide.'I'his contact, it is to be emphasized, is not the haphazard Vcontactbetween nite and separate particles of zinc oxide and carbon, as hasbeen employed in reduction processes prior hereto, but, on the contrary,it is an actual finite zinc oxide-'carbon contact existing within eachconcrete particle of zinc oxide, thus providing optimum conditions forthe rapid reduction of the zinc oxide when these pulverized zincoxide-coke briquettes are brought up to the required zones oftemperature for reduction.

-The charge of pulverized briquettematerial is passed into a preheaterconsisting of a rotary kiln which is ilred externally and in which atemperature of from about '100 C. to about 800 C. is maintained.

^ Reduction of the charge The charge, -prepared as above, and heated tofrom about 700 C. to about 800 C. (the temperature may vary according tothe nature and characteristics of the zinc oxide to be reduced), is

duction operations# with the exception that in In order to .effect theimprovements of the present process.' the purified sintered zinc oxideprepared'as'above, "together with coke, low-ash anthracite coal, orother carbonaceous material lowin mineral matter, and in the proportionof about 100 partsby weight of the zinc oxide sinter to from about-20vto 25 parts byweight of the low-ash vcarbonaceous reducing material,are' mixedtogethenthoroughly while water-free hot tar is introduced intothe mixture. the tar addicontinuously fed and showered into a verticalreducer furnace or retort Vcomposed of silicon carbide, maintained at atemperature of from about.1100 C. to about 1250" C., in which thereduction of the zinc oxide-carbon complex is eiected principallyaccording to the reaction:

zno+C'ozn+co2 1) Since, however. there is an excess of carbon at aglowing temperature in the furnace "charge, the producer-gas reactionbecomes eiective:

The value of the presence oi' carbon particles in intimate contact withthe zinc oxide of the sinterV is indicated by Reaction 2. A It is knownthat the speed of Reaction 1 is as possible. The carbon present withineach concrete particle of zincslnter offers immediate contact with theliberated C02.

This is not the case when the carbon is present in the charge only asparticles separate and distinct from the zinc calcine particles of thesusdenser for zinc maintained at the optimum temperature for thecondensation of the zinc vapor s to liquidzinc.

The residual carbon monoxide stream, carrying a small proportion of zincdust and blue powder,

passes through a small column through which flows counter-currentlyaspray of'water.

This collected zinc dust and blue powder mud- (zinc dust, zinc oxide,and blue powder) is returned to the pug mill for afurther treatment withthe following charge of purified sintered zinc oxide, while the washedcarbon monoxide issuing from the zinc condenser is utilized to provide acontrolled non-oxidizing atmosphere in both the coking briquette furnaceand the preheating rotary kiln for the charge of pulverized briquettesof zinc oxide-carbon. The carbon monoxide, after its passage throughboth furnaces, is burned to supply additional heat units in bothfurnaces.

In the flash reduction furnace, there may be provided an incandescentcolumn of coke, although the presence thereof is not obligatory by anymeans. But where such coke column is provided, its temperature is at thehigh heat level required (1200 C.1300 C.), which is maintained byindirect heating, as when adjacent to the flash reduction furnace wall,by electrical resistance,

or by direct partial combustion by the admittance at a point above thebottom of the coke column of a strictly metered amount of atmosphericair.

The sized coke (all through a Z-inch ring and plus 3A inch) is fed tothe coke column at a continuous rate through a gastight seal 'and thereis withdrawn at an equal rate and continuously an equal amount from thefoot of the .column also through a gas-tight seal, and quenched. Thecoke also may be supplied to the reduction furnace in the form ofbriquettes. J

As has been pointed out above, the present invention provides means formaintaining carbon in intimate contact with the zinc oxideof the charge;and it has been pointed out above that the reduction of the zinc oxide,proceeds in accordance with the two reactions:

zno+co+zn co2 k1) CO2+C 2CO f (2) producer reaction` g furnace isbrought to a temperature of 800 C.' 900 C. before introduction to theflash reduction metal.

coke that is being withdrawn continuously. The

withdrawn solids, coke, and residues, are screened,

all material passing a '3A-inch screen is separated,.

and the v-i-r'inch coke is returned to the feed of the coke column,while all V2-inch materialis crushed and passed to the feed of therotary kiln as reducing and heating fuel. The 9A-inch and +1/2-inchmaterial is sent to a crusher for addition as fuel to the reductionfurnace charge preparatory to the tar addition.

The coke fines also carry the residues from the zinc charge of thesuspension zinc reduction furnace. These nes pass to the rotary kiln andare mixed with the incoming charge of crude zinc ore calcines and coke.

This procedure enables the residual zinc values to be utilized, togetherwith the withdrawn undersized coke.

The purified zinc vapors and gases now consisting substantially whollyof carbon monoxide with a small percentage of nitrogen (from tue air,admitted in regulated amount to the base of the incandescent cokecolumn), pass 'to the condenser wherein the zinc vapor is condensed toliquid The residual gases, mainly C0, issuing from the scrubbing towerofthe zinc condenser, are returned in the volume required to the top ofthe flash reduction furnace to provide the carbon monoxideatmosphereequired for the zinc reduction process and arepreheated to1000" C.- 1100 C. before admission to the furnace. The admission of aregulated amount of air at the base of the incandescent coke columnserves the further purpose in helping to scavenge and evacuate therefromany zinc vapor that might have a tendency to concentrate at the bottomof the furnace within the,coke column. This function is in addition toits assistingl in maintaining the temperature of the coke column at thelevel desired.

After passage through the zinc condenser, the

residual gases and zinc vapors go through an adsettling in thecondenser. Any uncondensed zinc vapor is oxidized partly or wholly inthe scrubbing tower and is collected as a mud consisting of zinc oxideand zinc dust. The product is removed at intervals from the bottomsettling tank, which also forms a gas seal.

The clean residual gases, which consist principally of carbon monoxide,pass through a preheater, a portion thereof passing thence to' thepreheating furnace in which the' finely com' minuted charge of thesuspension zinc reduction furnace. l

The following products are obtained from the mixed pug mill in which thezinc oxide sinter is with coke'and hot anhydrous tar, as has beendescribed above, for further treatment in the flash furnace.

Product 3, above, passes to the rotary kiln for the recovery,of the zincand carbon contained '.cautions must be taken to assure the producer gasreaction proceeding in 'the iiash column through interaction of thecarbon ofthe charge with any carbon dioxide tending to be formed, which,however, cannot exist as such. Consequently, sufllcient carbon in thecharge must be provided to maintain the gas producer Reaction 2, above,for minimizingl blue powder formation. Also, of course, the eliminationof the coke column obviates introduction of air into the environment inthe flash reduction shaft. Otherwise, the operation is inaccordance-with the description given above. Y'

The following illustrative data show the results-obtained in thepreparation of the zinc 'oxide for charging.

A zinc sulphide concentrate assaying Zn 61.77 per cent; S 30.82-percent; Fe 1.15 .per cent; Pb 1.12 per cent; Cd 0.389 per cent; CaO 0.10per cent; MgO 0.594 per cent; SiO: 3.26 per cent;

A1203 0.51 per cent, first was roasted.

The roasted concentrate analyzed: Zn 71.30

per cent; S 2.55 per cent; Pb 0.31 per cent; Cd 0.03 per cent; Fe 1.69per cent; CaO 0.29 per cent; `Mg 0.70 per cent; S102 5.34 per cent;A1203 0.60 per cent; 017.45 per cent.

This roasted concentrate was' charged into the rotary kiln and the zincoxide produced by the kiln in a procedure similarto the well knownlWaelz process.

This zinc oxide analyzed; ZnO 87.0 per cent (Zn 69.8 per cent) S 2.0 percent; Pb0.03 per cent; Cd "0.31 per cent; SiOnJllzOa' 4.00 per cent;CaOMgO 0.50 percent; C 6.14 per cent.

This oxidlc material thenis sintered in the presence of salt, asdescribed above, prior to the mixing with tar in the pug mill. Thesintered product analyzed: ZnO 94.80 per cent; SiOzAlzOa Since carbonmonoxide is the principal reducing agent for the zinc, the use of thecarbon monoxideV atmosphere introduces no extraneous material into thereacting system; and with the intimate contact between the carbon andthe zinc oxide, which is provided by the present invention, .there areprovided optimum conditions for the maintenance of the producer gasreactions.

While the coke column may be omitted,`its

. use as described herein, is preferred.

' therein. The product is mixed with fresh incom- I claim:

1. The process of producing zinc, which coinprises converting an impurezinciferous material into vsubstantially pure zinc oxide free fromcadmium, lead. silver, and lall other materials chemically reactivetowards silicon carbide, preparing a charge of the said pure zinc oxideby intimately mixing it with a substantially anhydrous bituminousmaterial including water-free tar, kneading the tar until completeimpregnation with the tar is eiected, heating the resulting mixture to acoking temperature until the bituminous material is coked in situ in thezinc.

oxide, finely comminuting the resulting material, maintaining the cokeand oxide in continuous contact. showering the resulting mixture througha ash reduction environment having surfaces in contact with the materialcomposed of silicon carbide heated to reducing temperatures, andeiecting a reduction of the into a substantially pure zinc oxide freefromv materials chemically reactive towards silicon carbide, preparing acharge of the said pure zinc -oxide by intimately mixing it with abituminous material adapted to form coke upon heating to` cokingtemperature,l kneading together the bituminous material and the purezinc oxide until complete intermixing results, heating thev y resultingmixture to coking temperature until j' the bituminous material is cokedin situ in the zinc oxide, finely commuting the resulting materialmaintaining the coke and oxide in continuous contact. showering theresulting `mixture as a charge through a flash reduction environmenthaving surfaces in contact with the mixture composed of silicon carbideheated to reducing temperatures, and effecting a, reduction of thecharge to elemental zinc, while the charge is in free movement in theenvironment.

3. In the metallurgy of zinc, ther improvements which consist inproducing zinc oxide containing not more than substantially 0.6% ofcontaminants that are reactive towards silicon carbide at elevatedtemperatures intimately mixing and impregnating the resulting zinc oxidewith a -bituminous material high in fixed carbon and adapted to bind-thezinc oxide, by' kneading together the zinc oxide and bituminous materialinto an extrudible material, .extruding the resultingl mixture,briquetting the extruded mixture, coking 'the resulting briquettesthereby carbonizing the bituminous material, commmuting the briquettes,whereby each resulting particle is acomposite particle of zinc oxide andcarbon, and showering the resulting comminuted material through avertical'silicon carbide retort externally heated to a sufficiently hightemperature to reducethe zinc oxide to metallic zinc vapors while thepartiel are in free suspension.

4. The process of producing zinc, which coinprises converting an impurezinciferous material into substantially pure zinc oxide free fromcadmium, lead, silver, iron and substantiallyl all of any othermaterials chemically reactive to- -wards silicon carbide, preparing acharge of the. said pure zinc oxide by mixingit with a bitum'i-v u nousmaterial including tar, kneading the tar" ofthe zinc oxide with thev taris eifected, heating the resulting mixture to a coking temperaturematerial, maintaining the coke and oxide in continuous contact,showering the resulting into the zinc oxide until thorough impregnationmixture through a ash reduction environment having surfaces in contactwith the material Icomposed of silicon carbide heated to reducingtemperatures, and eiecting a reduction of the charge to elemental zincwhile the charge is infree movement inthe environment.'

AUGUSTIN LEON JEAN QUENEAU

